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Abstract

We report the realization of an open cavity whispering gallery mode optical resonator, in which the circulating light traverses a free space gap. We utilize focused ion beam microfabrication to precisely cut a 10 μm wide notch into the perimeter of a crystalline disc. We have shown that this modified resonator structure supports high quality modes, and demonstrated qualify factor, Q ≃ 106, limited by the notch surface roughness due to the ion milling process. Furthermore, we investigated the spatial profile of the modes inside the open cavity with a microfabricated probe mechanism. This new type of resonator structure facilitates interaction of the cavity’s optical field with mechanical resonators as well as individual atoms or molecules.

Figures (5)

SEM images (a,b) of CaF2 FIBED resonator with 50 μm thickness and 1.15 mm radius, and a photograph (c) from above after partial cleaning. Also shown is a close-up SEM image (d) of the 10 μm notch viewed at a slight angle.

SEM images of notch surfaces, including top view (a) after the initial coarse milling and the right wall partially milled with finer FIB precision. Images at ×10,000 (right) compare the two notch surfaces after many iterations of ion beam polishing. Left wall (b) exhibits 90 μm surface features while the right (c) has smoother surface finish than the SEM resolution (< 25 μm).

Diagram of the optical set-up (a) illustrating the two angle-polished fiber couplers and detectors for the transmitted forward (CW) and backward (CCW) light. A photograph with red light coupled (b) shows the fiber couplers and scattering from the notch on top.

Probing the optical modes with (a) 5μm-thick gold wire tip. This micro beam-block introduces losses as it is inserted radially (b) into the gap from just outside the perimeter. The mode amplitude plot (c) shows a mode (blue) attenuated within the first 4 μm of insertion depth, while another (red) exists deeper in the WGMR.